1. Field of the Invention
The present invention provides an electrochemical cell and subassembly for an electrochemical cell, particularly electrochemical stacks.
2. Description of the Related Art
Electrochemical cells include, but are not limited to, fuel cells, electrolysis or electrolyzer cell, electrochemical synthesis cells, electrochemical oxygen concentrators (also known as electrochemical oxygen pumps), and electrochemical hydrogen concentrators (also known as electrochemical hydrogen pumps). Electrochemical cell stacks are made up of individual electrochemical cells that are connected in series. The structure and operation of these electrochemical cells have many common aspects, which will be discussed herein in the context of a fuel cell stack.
The primary components of a proton exchange membrane (PEM) fuel cell stack are the membrane and electrode assemblies (MEAs), gas diffusion layers, and bipolar plate/flow field assemblies. These components are assembled in a “stack” with each gas diffusion layer/MEA/gas diffusion layer in the stack separated by a bipolar assembly and each end of the stack having an endplate. Conventionally, the stack is held together under compression, such as by threaded tie rods or a series of bands.
Each of the cells in the stack has an MEA made up of a cathode electrode in intimate contact with one side of a proton exchange membrane (PEM) and an anode electrode in intimate contact with the opposite side of the proton exchange membrane. In the case of a hydrogen consuming PEM fuel cell, the anode electrode comprises an electrocatalyst layer and a porous hydrophobic gas diffusion layer/backing layer. Similarly, the cathode electrode of the PEM fuel cell comprises an electrocatalyst layer and a porous hydrophobic gas diffusion layer/backing layer. For a typical PEM electrolysis cell, the anode electrode comprises an electrocatalyst layer and a porous substrate/current collector material. Similarly, the cathode electrode of a typical PEM electrolysis cell comprises an electrocatalyst layer and a porous substrate/current collector material.
The bipolar plate/flow field assemblies are located between adjacent gas diffusion layer/MEA/gas diffusion layer assemblies and provide flow fields or chambers through which reactants are channeled across the face of each electrode while maintaining separation of the reactants and products. The bipolar plate/flow field assemblies and, where applicable, gas diffusion structures that provide support and backing to the electrocatalyst layers, serve to conduct electricity between each of the cells in the stack and further to ensure that the reactant fluids are evenly distributed over the active portions of the electrodes. Additionally, bipolar plates may be constructed with internal channels to allow the use of cooling fluids for cooling the electrochemical stack if desired.
An assembled electrochemical cell stack can become quite large and heavy with most of the weight being associated with the bipolar plate/flow field assemblies. A well-known challenge to broad usage of PEM fuel cell stacks is that the weight of the stack can become so great that the corresponding power-to-weight ratio becomes unacceptably low. Therefore, many designs of the bipolar plate/flow field assemblies have been developed to reduce the overall weight of the stacks and increase the power-to-weight ratios. These designs have included, for example, use of metal foam in the construction of bipolar plate/flow field assemblies, as described in U.S. Pat. No. 6,146,780 to Cisar et al., and hereby incorporated by reference, and use of low-density metals that have been coated with a thin layer of a more noble metal for protection against corrosion, as described in U.S. Pat. No. 6,203,936 to Cisar et al., and hereby incorporated by reference in its entirety.
While these reduced-weight bipolar plate/flow field assemblies have resulted in electrochemical fuel cell stacks having good power-to-weight ratios when the current densities are greater than 400 mA/cm2, these designs are not as effective to provide efficient fuel cell stacks when the operating pressure of the reactants is low, such as in a fuel cell using ambient air on aircraft flying at high altitudes.
What is needed is a fuel cell stack that provides a lightweight bipolar assembly for an electrochemical cell stack. It would be desirable if the electrochemical cell stack yielded an improved power-to-weight ratio. It would also be desirable if the fuel cell would operate well at low reactant pressures, i.e., at pressures less than one atmosphere. It would be of further benefit if the electrochemical cell could operate with minimal pressure drop through the airside flow field to maintain as high a pressure throughout the system as possible.